The present invention relates to vinyl aromatic polymer compositions having improved impact strength properties. Further, the present invention relates to a process for preparing the said compositions by mixing of the different constituents.
It is well known that resins obtained from polystyrene as sole constituent have a low impact strength. In order to improve that property, the polystyrene which is hard and rigid, is combined with a rubber which is flexible and elastic. Two processes may be utilized to combine the polystyrene and rubber. One of these processes involves physically mixing polystyrene and rubber. However, the compatibility of the polystyrene and rubber is low and generally, mixing results in the rubber being poorly dispersed in the polystyrene. Rubber agglomerates are more or less finely divided into the polystyrene mass, and the properties of the polystyrene are only slightly improved. In order to increase the impact strength by mixing polystyrene and rubber, a high rubber content is needed. However, in adding high rubber amounts, other desirable properties of polystyrene are unfavorably influenced.
The other method of combining rubber and polystyrene is by grafting styrene on rubber. This method, which often comprises a bulk polymerization followed by a suspension polymerization, requires high investments. Moreover, the grafting process has limitations particularly with respect to the density and the quality of the grafting.
Compositions comprising a vinylaromatic polymer, 1,4 polybutadiene and a statistic polymer of butadiene and styrene (such as GRS) are also known. However, the impact strength properties of such compositions are not as good as is often desired. Other compositions of thermoplastic resins obtained by mixing a homopolymer or a copolymer of styrene with a sequenced copolymer of styrene and butadiene are known, but the impact strength properties of these compositions also do not meet many needs. The feature of this type of composition resides in the fact that the whole rubber present in the resin is grafted. However, if a maximum degree of graft improves tensile strength and bending strength properties, the results show that it harms the impact strength properties.
It is an object of the present invention to provide new and improved high impact polystyrene compositions.
Another object of the present invention is to provide a new and improved high impact polystyrene prepared by mixing.
Still another object of the present invention is to provide a process for mixing the materials necessary to prepare the new and improved high impact polystyrene compositions defined herein.
Additional objects will become apparent from the following description of the invention herein described.